This is directly in my field, so feel free to AMA about this. As a disclaimer, I am not entirely convinced of the interpretation, but I do believe the data is showing that the standard...
Exemplary
This is directly in my field, so feel free to AMA about this. As a disclaimer, I am not entirely convinced of the interpretation, but I do believe the data is showing that the standard cosmological model, Lambda-CDM, is not adequately explaining the expansion history of the universe anymore. I am not sure that evolving dark energy is the only or best explanation (although it could be).
There are a lot. This would be the first clue we have gotten in the ~30 years since dark energy was discovered that tells us something about what it is. The implications of that are massive, as...
There are a lot. This would be the first clue we have gotten in the ~30 years since dark energy was discovered that tells us something about what it is. The implications of that are massive, as currently dark energy doesn't really fit into the standard model of particle physics, so with some new evidence about what it is, we might finally be able to make some progress there.
The specific implications are confusing. The type of evolving dark energy that the DESI data seems to prefer exhibits something called "phantom crossing", which means it violates something known as the null energy condition. This essentially says that the energy density of things cannot increase with cosmic expansion: at best, they can remain constant (like a cosmological constant would), or they can decrease (as do the densities of matter and radiation). Phantom crossing poses serious theoretical challenges. There are some theories out there that exhibit phantom crossing, but the majority don't. Most people expect/expected the null energy condition to not be violated, although there is nothing hard or fast that says this must be so. This is part of the reason I am skeptical of the evolving dark energy interpretation.
The null energy condition is one of the assumptions that goes into Penrose's singularity theorems, which is the theorem that guarantees black holes have a singularity at their center.
What is your favorite explanation for dark energy? Not necessarily the one that you think it is, just the one that you would like to see turn out to be correct (if there is one). If I'm not...
What is your favorite explanation for dark energy? Not necessarily the one that you think it is, just the one that you would like to see turn out to be correct (if there is one). If I'm not mistaken, my cosmology professor hoped it was a fifth fundamental force.
I find the most elegant and exciting explanation to be that it is the cosmological constant, i.e. the energy of empty space itself. While this is what is actually assumed in the standard...
I find the most elegant and exciting explanation to be that it is the cosmological constant, i.e. the energy of empty space itself. While this is what is actually assumed in the standard cosmological model, Lambda-CDM (the Lambda stand for the cosmological constant, the CDM stands for cold dark matter), the interpretation faces serious theoretical roadblocks.
Thanks for posting this. I intend to read the article but I wanted to ask how this would relate to evidence for a MOND-like universe, as discussed by McGaugh et al. . I was very taken with the...
Thanks for posting this. I intend to read the article but I wanted to ask how this would relate to evidence for a MOND-like universe, as discussed by McGaugh et al. . I was very taken with the arguments in the paper and wonder if the data in the Nature paper points in the same direction.
I'm somewhat familiar with Stacy McGaugh's arguments. I personally am not convinced for various reasons. The analyses in the DESI papers recently released assume General Relativity to do their...
I'm somewhat familiar with Stacy McGaugh's arguments. I personally am not convinced for various reasons.
The analyses in the DESI papers recently released assume General Relativity to do their analysis. However, data from the Atacama Desert Telescope (ACT), also released this week, when combined with data from DESI (not the most recent release) has found evidence for a deviation from GR that is at around the 3 sigma level. That means there's about a 3% chance of it being a statistical fluke. While that may sound unlikely, you must remember how many different things the collaboration tests for, and consider how likely it is at least one of those tests shows some deviation (this is known as the look-elsewhere effect). A 3 sigma deviation certainly gets attention, but it is not taken as strong evidence for or against anything.
I got burned by pathological science during my postdoc, resulting in my ensuing departure from academia (for which I am now grateful). Statistical significance is what separates fact from fantasy.
I got burned by pathological science during my postdoc, resulting in my ensuing departure from academia (for which I am now grateful). Statistical significance is what separates fact from fantasy.
What is unique about this survey that it is spotting something different than previous observations? And what are your thoughts on the Atacama survey seeming to confirm the opposite (I tagged you...
What is unique about this survey that it is spotting something different than previous observations?
And what are your thoughts on the Atacama survey seeming to confirm the opposite (I tagged you in that comment)?
DESI is a really amazing instrument. In order to make these measurements, they need to essentially create a 3D map of the universe, measuring the angular position of galaxies on the sky along with...
DESI is a really amazing instrument. In order to make these measurements, they need to essentially create a 3D map of the universe, measuring the angular position of galaxies on the sky along with their redshift. Redshift refers to how much the spectrum of the light you are receiving has been shifted from where it would be if observed in the lab, due to the expansion of the universe (you can think of it as the light equivalent of the Doppler effect, where the frequency of light changes because these galaxies are receding from us). It is a measure of how much the universe has expanded since the light was emitted, and is related to the distance to the object, but is much easier to measure.
Measuring the angular position of galaxies on the sky is easy. Measuring the redshift is easy, but it takes time, since you have to observe a galaxy for a longer exposure in order to measure its spectrum (instead of just measuring the intensity in a few color filters). The main innovation of DESI is it's focal plane, which contains over 5000 optical fibers that can be individually, robotically pointed. When DESI looks at a patch of the sky, each of these fibers is focused on a different galaxy in the patch, and measures its spectrum. This allows DESI to significantly increase the number of spectra it can record, because while doing so still takes a long exposure, it can knock out 5000 galaxies as once. In just one year of taking data, it matched the precision of all preceding comparable galaxy surveys, combined. The newest data that was just released represents 3 years of collecting data, and is therefore much more precise.
I think quantifying the time dilation will probably clear up a lot of confusion. It's hard to pin down dark energy when time dilation is muddling every value we measure that comes from...
I think quantifying the time dilation will probably clear up a lot of confusion. It's hard to pin down dark energy when time dilation is muddling every value we measure that comes from sufficiently far away. If we collect enough standard candle data perhaps we can finally account for the tidi, get it out of the measurements, and get a clear, accurate look at pure expansion data. Then we can put L-CDM into a death match with its contemporaries and see who wins.
Look on the bright side - heat death is postponed greatly, if not indefinitely. That's more time to work on the problem! :)
The "timescape" theory in the video you linked to is not well accepted because it does not fit the data any better than LCDM, and there is no evidence that its central claim is true. It's an...
The "timescape" theory in the video you linked to is not well accepted because it does not fit the data any better than LCDM, and there is no evidence that its central claim is true. It's an example of a inhomogeneous cosmology which posits that the inhomogeneities in the distribution of matter that we see are significant enough that they cannot be averaged over and the FRW metric that underpins LCDM and its extensions is not valid. These theories are not genuinely fringe but they are just on the other side of the divide.
For these theories to be true, inhomogeneities must be vastly larger than we measure, time dilation effects must be orders of magnitude larger than we calculate, and even then it turns out that these inhomogeneities tend to deccelerate the expansion of the universe rather than accelerate. While timescape theories can fit supernovae data alright, they fail at fitting other probes across a range of scales.
But even as the DESI observations challenged the standard model of cosmology, which predicts that dark energy is constant across time, a separate result has reinforced it. On Tuesday, the multinational team that ran the Atacama Cosmology Telescope in Chile released the most detailed images ever taken of the infant universe, when it was a mere 380,000 years old. (That telescope was decommissioned in 2022.)
Their report, not yet peer-reviewed, seems to confirm that the standard model was operating as expected in the early universe. One element in that model, the Hubble constant, describes how fast the universe is expanding, but over the last half-century measurements of the constant have starkly disagreed, an inconsistency that today has shrunk to about nine percent. Theorists have mused that perhaps an additional spurt of dark energy in the very early universe, when conditions were too hot for atoms to form, could resolve this so-called Hubble tension.
The latest Atacama results seem to rule out this idea. But they say nothing about whether the nature of dark energy might have evolved later in time.
Both reports evoked effusive praise from other cosmologists, who simultaneously confessed to a cosmic confusion about what it all meant.
ACT observes the cosmic microwave background, which can be thought of as a snapshot of the primordial plasma that existed immediately after the Big Bang, when the universe was so hot and dense...
ACT observes the cosmic microwave background, which can be thought of as a snapshot of the primordial plasma that existed immediately after the Big Bang, when the universe was so hot and dense that photons and normal matter existed in a plasma much like the plasma of the sun. As such, we learn mostly about the early universe (the first 300k years or so) by studying the CMB (this is actually what I do), with some information about the late universe included because photons are gravitationally lensed as they travel to us.
The DESI measurements, on the other hand, observe galaxies that took a while to form, and therefore exist in the "late universe". By late universe I mean the last few billion years or so. When doing galaxy surveys like this, we therefore primarily learn about the late universe, but to do so we have to calibrate a few things based on our understanding and measurements of the early universe from the CMB. Dark energy, evolving or not, only becomes a significant effect in the last ~4 billion years, so we don't learn about it from the CMB. Therefore it is possible that ACT observations are broadly consistent of the standard model, where dark energy is treated as a constant, since for the most part if it wasn't constant it simply wouldn't show up in that data.
The connections between the two sets of measurements (i.e. the need to calibrate the late-time stuff with early time stuff, or the influence of late-time stuff on the the stuff we see from early times from gravitational lensing) complicates the matter and sorting out the details is what everyone is focused on now.
This is directly in my field, so feel free to AMA about this. As a disclaimer, I am not entirely convinced of the interpretation, but I do believe the data is showing that the standard cosmological model, Lambda-CDM, is not adequately explaining the expansion history of the universe anymore. I am not sure that evolving dark energy is the only or best explanation (although it could be).
What are the most interesting implications of this?
There are a lot. This would be the first clue we have gotten in the ~30 years since dark energy was discovered that tells us something about what it is. The implications of that are massive, as currently dark energy doesn't really fit into the standard model of particle physics, so with some new evidence about what it is, we might finally be able to make some progress there.
The specific implications are confusing. The type of evolving dark energy that the DESI data seems to prefer exhibits something called "phantom crossing", which means it violates something known as the null energy condition. This essentially says that the energy density of things cannot increase with cosmic expansion: at best, they can remain constant (like a cosmological constant would), or they can decrease (as do the densities of matter and radiation). Phantom crossing poses serious theoretical challenges. There are some theories out there that exhibit phantom crossing, but the majority don't. Most people expect/expected the null energy condition to not be violated, although there is nothing hard or fast that says this must be so. This is part of the reason I am skeptical of the evolving dark energy interpretation.
The null energy condition is one of the assumptions that goes into Penrose's singularity theorems, which is the theorem that guarantees black holes have a singularity at their center.
What is your favorite explanation for dark energy? Not necessarily the one that you think it is, just the one that you would like to see turn out to be correct (if there is one). If I'm not mistaken, my cosmology professor hoped it was a fifth fundamental force.
I find the most elegant and exciting explanation to be that it is the cosmological constant, i.e. the energy of empty space itself. While this is what is actually assumed in the standard cosmological model, Lambda-CDM (the Lambda stand for the cosmological constant, the CDM stands for cold dark matter), the interpretation faces serious theoretical roadblocks.
Do you have a favored solution to the problem?
Nope!
Thanks for posting this. I intend to read the article but I wanted to ask how this would relate to evidence for a MOND-like universe, as discussed by McGaugh et al. . I was very taken with the arguments in the paper and wonder if the data in the Nature paper points in the same direction.
I'm somewhat familiar with Stacy McGaugh's arguments. I personally am not convinced for various reasons.
The analyses in the DESI papers recently released assume General Relativity to do their analysis. However, data from the Atacama Desert Telescope (ACT), also released this week, when combined with data from DESI (not the most recent release) has found evidence for a deviation from GR that is at around the 3 sigma level. That means there's about a 3% chance of it being a statistical fluke. While that may sound unlikely, you must remember how many different things the collaboration tests for, and consider how likely it is at least one of those tests shows some deviation (this is known as the look-elsewhere effect). A 3 sigma deviation certainly gets attention, but it is not taken as strong evidence for or against anything.
I got burned by pathological science during my postdoc, resulting in my ensuing departure from academia (for which I am now grateful). Statistical significance is what separates fact from fantasy.
What is unique about this survey that it is spotting something different than previous observations?
And what are your thoughts on the Atacama survey seeming to confirm the opposite (I tagged you in that comment)?
DESI is a really amazing instrument. In order to make these measurements, they need to essentially create a 3D map of the universe, measuring the angular position of galaxies on the sky along with their redshift. Redshift refers to how much the spectrum of the light you are receiving has been shifted from where it would be if observed in the lab, due to the expansion of the universe (you can think of it as the light equivalent of the Doppler effect, where the frequency of light changes because these galaxies are receding from us). It is a measure of how much the universe has expanded since the light was emitted, and is related to the distance to the object, but is much easier to measure.
Measuring the angular position of galaxies on the sky is easy. Measuring the redshift is easy, but it takes time, since you have to observe a galaxy for a longer exposure in order to measure its spectrum (instead of just measuring the intensity in a few color filters). The main innovation of DESI is it's focal plane, which contains over 5000 optical fibers that can be individually, robotically pointed. When DESI looks at a patch of the sky, each of these fibers is focused on a different galaxy in the patch, and measures its spectrum. This allows DESI to significantly increase the number of spectra it can record, because while doing so still takes a long exposure, it can knock out 5000 galaxies as once. In just one year of taking data, it matched the precision of all preceding comparable galaxy surveys, combined. The newest data that was just released represents 3 years of collecting data, and is therefore much more precise.
I think quantifying the time dilation will probably clear up a lot of confusion. It's hard to pin down dark energy when time dilation is muddling every value we measure that comes from sufficiently far away. If we collect enough standard candle data perhaps we can finally account for the tidi, get it out of the measurements, and get a clear, accurate look at pure expansion data. Then we can put L-CDM into a death match with its contemporaries and see who wins.
Look on the bright side - heat death is postponed greatly, if not indefinitely. That's more time to work on the problem! :)
The "timescape" theory in the video you linked to is not well accepted because it does not fit the data any better than LCDM, and there is no evidence that its central claim is true. It's an example of a inhomogeneous cosmology which posits that the inhomogeneities in the distribution of matter that we see are significant enough that they cannot be averaged over and the FRW metric that underpins LCDM and its extensions is not valid. These theories are not genuinely fringe but they are just on the other side of the divide.
For these theories to be true, inhomogeneities must be vastly larger than we measure, time dilation effects must be orders of magnitude larger than we calculate, and even then it turns out that these inhomogeneities tend to deccelerate the expansion of the universe rather than accelerate. While timescape theories can fit supernovae data alright, they fail at fitting other probes across a range of scales.
NYT report (no paywall)
@gpl
ACT observes the cosmic microwave background, which can be thought of as a snapshot of the primordial plasma that existed immediately after the Big Bang, when the universe was so hot and dense that photons and normal matter existed in a plasma much like the plasma of the sun. As such, we learn mostly about the early universe (the first 300k years or so) by studying the CMB (this is actually what I do), with some information about the late universe included because photons are gravitationally lensed as they travel to us.
The DESI measurements, on the other hand, observe galaxies that took a while to form, and therefore exist in the "late universe". By late universe I mean the last few billion years or so. When doing galaxy surveys like this, we therefore primarily learn about the late universe, but to do so we have to calibrate a few things based on our understanding and measurements of the early universe from the CMB. Dark energy, evolving or not, only becomes a significant effect in the last ~4 billion years, so we don't learn about it from the CMB. Therefore it is possible that ACT observations are broadly consistent of the standard model, where dark energy is treated as a constant, since for the most part if it wasn't constant it simply wouldn't show up in that data.
The connections between the two sets of measurements (i.e. the need to calibrate the late-time stuff with early time stuff, or the influence of late-time stuff on the the stuff we see from early times from gravitational lensing) complicates the matter and sorting out the details is what everyone is focused on now.